1. Field of the Invention:
The present invention relates to a refrigerator oil composition. More precisely, it relates to a refrigerator oil composition containing a refrigerant that comprises carbon dioxide (CO.sub.2) as the essential component, and to a method of using the composition for lubrication especially in a compression refrigeration cycle equipped with an oil separator and/or a hot gas line.
2. Description of the Related Art:
In general, a compression refrigeration cycle for refrigerators, for example, that for compression refrigerators comprising a compressor, a condenser, an expansion valve and an evaporator is of a closed system in which is circulated a mixed liquid comprising a refrigerant and a lubricating oil. In compression refrigerators of that type, in general, chlorofluorocarbons such as dichlorodifluoromethane (R-12) and chlorodifluoromethane (R-22) have heretofore been used as refrigerants. Various types of lubricating oils have been produced and used together with such refrigerants. However, when released in air, these Flon compounds that have heretofore been used as refrigerants will bring about environmental pollution, as destroying the ozone layer existing in the stratosphere. Therefore, their use is being severely controlled in all the world. Given the situation, new refrigerants, hydrogen-containing Flon compounds such as hydrofluorocarbons and hydrochlorofluorocarbons have become specifically noted. Hydrogen-containing Flon compounds, especially hydrofluorocarbons such as typically 1,1,1,2-tetrafluoroethane (R-134a) will not destroy the ozone layer. However, as their life in air is long, they will cause global warming. Therefore, the recent tendency in the art is toward using refrigerants from natural resources that are free from the problem.
Above all, carbon dioxide (CO.sub.2) is excellent as it is harmless to the environment and is safe to human beings. In addition, it has the advantages of (1) appropriate pressure near to the optimum pressure in economical aspects, (2) extremely small pressure ratio as compared with other conventional refrigerants, (3) good compatibility with ordinary oil and with construction materials for machines, (4) anywhere easy availability, and (5) extremely low costs with no necessity for recovery. However, using such CO.sub.2 as a refrigerant is often problematic in that, when it is combined with ordinary refrigerator lubricating oil, the oil composition has poor lubricity and its ability to improve the abrasion resistance of machine parts is poor. As a result, the machine parts being lubricated with the oil composition will become unstable and could not keep long-term durability. In addition, the system in which is used CO.sub.2 requires higher jetting pressure, as compared with that where is used R-134a or the like, with the result that the viscosity of the lubricating oil therein is lowered and the sealability of system will be worsened.